January 8, 1998

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

What in the world is the Hilo Ridge?

Okay, all you Big Island residents, where
is the Hilo Ridge? It stands nearly 3,000 m (10,000 ft) above
its surroundings, is more than 50 km (30 mi) long, and is one
of the most prominent landmarks in the State. Its crest is only
30 km (20 mi) from Hilo--but northeast of town. Yes, the
Hilo Ridge is a feature of the seafloor, out of sight under the
waves but nonetheless an important piece in the puzzle of how
the Big Island was made.

The Hilo Ridge starts at the east edge
of a gentle shelf or platform about 400 m (1,300 ft) deep off
the Hamakua coast, about 20 km (12 mi) east of Honomu. From the
platform, the ridge extends eastward in a slightly sinuous trace,
finally ending at a depth of about 5,000 m (16,000 ft) some 65
km (37 mi) east-northeast of Hilo.

Until recently, earth scientists thought
that the Hilo Ridge was built by eruptions along a rift zone of
Mauna Kea, much like ridges along the rift zones that radiate
from the summits of Kilauea and Mauna Loa. Just as Kilauea's
east rift zone extends far beyond the shoreline as the Puna Ridge,
so the east rift zone of Mauna Kea was thought to have reached
far out to sea as the Hilo Ridge. The on-land part of Mauna Kea's
east rift zone today is barely recognizable, because younger flows
have covered those erupted along the rift zone. These younger
flows, however, did not reach far eastward beyond the coast, and
so the submarine rift zone remained a prominent topographic ridge.

In a startling development at a meeting
of the American Geophysical Union last month, researchers from
the University of Washington (including a former HVO staff member,
Robin Holcomb) proposed a new hypothesis for the Hilo Ridge. They
suggest that the ridge was built along a rift zone of the older
Kohala Volcano, not of Mauna Kea. In their view, Mauna Kea buries
and obscures the Kohala rift zone on the Big Island itself but
does not reach far enough eastward to cover the Hilo Ridge. What
is the evidence for such a radical departure from past interpretations?

The University of Washington group determined
the chemical compositions of rocks recently dredged from depths
of 5,000 m (16,000 ft), 1,100 m (3,600 ft), and 400 m (1,300 ft)
along the Hilo Ridge. They compared their results with those obtained
by previous workers on dredged samples from depths of 1,600-3,300
m (5,200-10,800 ft). The results show a pattern that is
inconsistent with the known chemical characteristics for Mauna
Kea, particularly in terms of the abundance of certain isotopes
of the elements strontium, neodymium, and lead. The data are,
however, entirely consistent with the known chemical compositions
of Kohala, an older volcano that stopped erupting some 120,000
years ago.

Kohala is a poorly understood volcano.
One of the problems is that a large part of the volcano, south
of a line between Kukuihaele, Waimea, and Kawaihae, is covered
by younger lava flows from Mauna Kea. The Kohala Mountains themselves
may be only the remnant of a northwest rift zone of Kohala Volcano
that sprouts from a buried caldera southeast of Waimea. It is
not difficult to imagine that Kohala's southeast rift zone
curves gently southward from the buried caldera and eventually
becomes the Hilo Ridge, just as Mauna Loa's southwest rift
zone and Kilauea's east rift zone bend away from their
respective calderas.

Will this stimulating new view of the
Hilo Ridge stand the test of time? As with any new idea in science,
it will be closely scrutinized over the next few years as new
information is obtained and discussed. The new idea certainly
passes the laugh test, but will it pass the fact test?

Eruption and Earthquake Update, 8 January

During the past week, there was constant
effusion of lava from the vent within Pu`u `O`o. Southerly winds
carried the gas emissions from the volcano as far north as Honolulu.
The concentration of sulfur dioxide measured in the parking lot
of HVO at times exceeded 5 parts per million (ppm), and several
visitor facilities within Hawaii Volcanoes National Park were
closed.

Lava continued to flow through a network
of tubes down to the seacoast where it entered the ocean at two
locations - Waha`ula and Kamokuna. Several minor breakouts from
the tube system produced surface flows. The public is reminded
that the ocean entry areas are extremely hazardous, with explosions
accompanying frequent collapses of the lava delta. The steam
cloud is highly acidic and laced with glass particles.

An earthquake at 5:35 p.m. on December
29 was reported felt by a resident of Pahala. The magnitude 3.8
earthquake was located 11.5 km (7 mi) southeast of the summit
of Kilauea Volcano at a depth of 7 km (4 mi).